Cream butter and sugar together. Stir in egg, lemon juice and vanilla. Sift in the flour. Line cookie sheet with greased waxed paper. Drop small teaspoonfuls in mounds about an inch apart on waxed paper. Dent each mound with your finger. Cut with paring knife small portion of jelly and fill in each dent. Bake 1 hour or until brown at 225 degrees. Makes 45 cookies.